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(#) Wrong Thread

!!! ERROR: Wrong Thread
   This is an error.

Id
:   `WrongThread`
Summary
:   Wrong Thread
Severity
:   Error
Category
:   Correctness
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   3.1.0 (March 2018)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/guide/components/processes-and-threads.html#Threads
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/ThreadDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ThreadDetectorTest.kt)

Ensures that a method which expects to be called on a specific thread,
is actually called from that thread. For example, calls on methods in
widgets should always be made on the UI thread.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/ThreadTest.java:15:Error: Method onPreExecute must be
called from the main thread, currently inferred thread is worker thread
[WrongThread]
    onPreExecute(); // ERROR
    --------------
src/test/pkg/ThreadTest.java:16:Error: Method paint must be called from
the UI thread, currently inferred thread is worker thread [WrongThread]
    view.paint(); // ERROR
    ------------
src/test/pkg/ThreadTest.java:22:Error: Method publishProgress must be
called from the worker thread, currently inferred thread is main thread
[WrongThread]
    publishProgress(); // ERROR
    -----------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/ThreadTest.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import androidx.annotation.MainThread;
import androidx.annotation.UiThread;
import androidx.annotation.WorkerThread;

public class ThreadTest {
    public static AsyncTask testTask() {

        return new AsyncTask() {
            final CustomView view = new CustomView();

            @Override
            protected void doInBackground(Object... params) {
                onPreExecute(); // ERROR
                view.paint(); // ERROR
                publishProgress(); // OK
            }

            @Override
            protected void onPreExecute() {
                publishProgress(); // ERROR
                onProgressUpdate(); // OK
                // Suppressed via older Android Studio inspection id:
                //noinspection ResourceType
                publishProgress(); // SUPPRESSED
                // Suppressed via new lint id:
                //noinspection WrongThread
                publishProgress(); // SUPPRESSED
                // Suppressed via Studio inspection id:
                //noinspection AndroidLintWrongThread
                publishProgress(); // SUPPRESSED
            }
        };
    }

    @UiThread
    public static class View {
        public void paint() {
        }
    }

    public static class CustomView extends View {
        @Override public void paint() {
        }
    }

    public abstract static class AsyncTask {
        @WorkerThread
        protected abstract void doInBackground(Object... params);

        @MainThread
        protected void onPreExecute() {
        }

        @MainThread
        protected void onProgressUpdate(Object... values) {
        }

        @WorkerThread
        protected final void publishProgress(Object... values) {
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ThreadDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `ThreadDetector.testThreading`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("WrongThread")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("WrongThread")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection WrongThread
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="WrongThread" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'WrongThread'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore WrongThread ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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